An electrographic image forming apparatus is widely used. Compared to an image forming apparatus of other systems, such as an inkjet system, a high quality image can be obtained in a short time.
In an image forming apparatus, an image is formed on a surface of a medium such as paper. In the image forming step, after an electrostatic latent image is adhered to a surface of a photosensitive drum, developer is adhered to the electrostatic latent image. Since the developer adhered to the electrostatic latent image is transferred to the surface of the medium and then heated and pressured, it is fused to the medium.
Developer includes an external additive as well as a coloring agent. The external additive functions to suppress agglomeration of developers.
Since a configuration of developer affects quality of an image, various considerations have been made relating to the configuration of the developer. Specifically, to improve the transferability, etc., of the developer, in cases where an external additive is fused to a mother particle, the average circularity degree of the mother particle is optimized (For example, see Patent Document 1).